1. Field of the Invention
The present invention relates to an auxiliary power supply unit for a portable electronic device, and also relates to a portable electronic system which is composed of a portable electronic device, such as a digital camera, and one or more associated accessories.
2. Description of the Related Art
Various types of batteries are available on the market; each type generally has a factory-recommended final voltage which guarantees the safety of the battery. If battery continues to be used after the battery voltage drops below the final voltage, the battery may produce heat, which in turn may leak liquid, or worse explode, under some conditions. If battery leaks or explodes, a device in which the battery is loaded may malfunction or may be damaged.
To prevent such problems from occurring, in conventional portable electronic devices which use a battery as a power supply, the battery voltage is regularly checked to ensure the safety of the battery. For example, if the battery voltage reaches the final voltage thereof, the user can be warned that the battery is nearly exhausted, and the user is either prompted to replace the battery or the operation of the portable electronic device is stopped to prohibit the user from further using the nearly-exhausted battery.
However, the internal resistance of the battery varies in accordance with the servicing environment of the battery and the operating condition of the portable electronic device. When the portable electronic device operates at a low temperature or when the portable electronic device is performing an operation requiring a large current sporadically, the internal resistance of the battery appears to increase, which may cause a power-supply-voltage detecting circuit provided in the device to mistakenly determine that the battery voltage has reached the final voltage. In this case, the user is erroneously informed that the battery is nearly exhausted, and therefore he or she replaces the battery with a new one. Consequently, the energy of the battery is not used up sufficiently.
To prevent this problem from occurring, a battery which excels in temperature characteristics can be used, a high precision power-supply-voltage detecting circuit can be provided in the portable electronic device, or the portable electronic device can be provided therein with an additional circuit which can reduce the internal resistance of the battery. However, all of these proposals cause the portable electronic device to be increased in size, thereby increasing weight and increasing the cost of production, which are undesirable. In addition, such proposals can be wasteful since, in some cases, the final voltage is hardly ever mistakenly detected under some conditions.